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The temperature structure of a giant planet was traditionally thought to be an adiabat assuming convective mixing homogenizes entropy. The only in-situ measurement made by the Galileo Probe detected a near-adiabatic temperature structure within one of Jupiter's 5$\mu$m hot spots with small but definite local departures from adiabaticity. We analyze Juno's microwave observations near Jupiter's equator (0 ~ 5$^o$N) and find that the equatorial temperature structure is best characterized by a stable super-adiabatic temperature profile rather than an adiabatic one. Water is the only substance with sufficient abundance to alter the atmosphere's mean molecular weight and prevent dynamic instability if a super-adiabatic temperature gradient exists. Thus, from the super-adiabaticity, our results indicate a water concentration (or the oxygen to hydrogen ratio) of about 4.9 times solar with a possible range of 1.5 ~ 8.3 times solar in Jupiter's equatorial region., Comment: 46 pages, 11 figures, accepted by Icraus

Water and ammonia vapors are known to be the major sources of spectral absorption at pressure levels observed by the microwave radiometer (MWR) on Juno. However, the brightness temperatures and limb darkening observed by the MWR at its longest wavelength channel of 50 cm (600 MHz) in the first 9 perijove passes indicate the existence of an additional source of opacity in the deep atmosphere of Jupiter (pressures beyond 100 bar). The absorption properties of ammonia and water vapor, and their relative abundances in Jupiter's atmosphere do not provide sufficient opacity in deep atmosphere to explain the 600 MHz channel observation. Here we show that free electrons due to the ionization of alkali metals, i.e. sodium, and potassium, with sub-solar metallicity [M/H] (log based 10 relative concentration to solar) in the range of [M/H] = -2 to [M/H] = -5 can provide the missing source of opacity in the deep atmosphere. If the alkali metals are not the source of additional opacity in the MWR data, then their metallicity at 1000 bars can only be even lower. The upper bound of -2 on the metallicity of the alkali metals contrasts with the other heavy elements -- C, N, S, Ar, Kr, and Xe -- which are all enriched relative to their solar abundances having a metallicity of approximately +0.5., Comment: This manuscript has been accepted for publication in The Astrophysical Journal Letters. The final version of the paper will be available in the published journal. This arXiv version is provided for informational purposes

The thermal structure of planetary atmospheres is an essential input for predicting and retrieving the distribution of gases and aerosols, as well as the bulk chemical abundances. In the case of Jupiter, the temperature at a reference level - generally taken at 1 bar - serves as the anchor in models used to derive the planet's interior structure and composition. Most models assume the temperature measured by the Galileo probe (Seiff et al. 1998). However, those data correspond to a single location, an unusually clear, dry region, affected by local atmospheric dynamics. On the other hand, the Voyager radio occultation observations cover a wider range of latitudes, longitudes, and times (Lindal et al. 1981). The Voyager retrievals were based on atmospheric composition and radio refractivity data that require updating and were never properly tabulated: the few existing tabulations are incomplete and ambiguous. Here, we present a systematic electronic digitization of all available temperature profiles from Voyager, followed by their reanalysis, employing currently accepted values of the abundances and radio refractivities of atmospheric species. We find the corrected temperature at the 1 bar level to be up to 4 K greater than previously published values, i.e., 170.3{\pm}3.8 K at 12{\deg}S (Voyager 1 ingress) and 167.3{\pm}3.8 K at 0{\deg}N (Voyager 1 egress). This is to be compared with the Galileo probe value of 166.1{\pm}0.8 K at the edge of an unusual feature at 6.57{\deg}N. Altogether, this suggests that Jupiter's tropospheric temperatures may vary spatially by up to 7 K between 7{\deg}N and 12{\deg}S., Comment: 11 pages, 3 figures

The Cassini mission to the Saturn system discovered a plume of ice grains and water vapor erupting from cracks on the icy surface of the satellite Enceladus. This moon has a global ocean in contact with a rocky core beneath its icy exterior, making it a promising location to search for evidence of extraterrestrial life in the solar system. The previous detection of H$_2$ in the plume indicates that there is free energy available for methanogenesis, the metabolic reaction of H$_2$ with CO$_2$ to form methane and water. Additional metabolic pathways could provide sources of energy in Enceladus' ocean, but require the use of other oxidants that have not been detected in the plume. Here, we perform chemical modeling to determine how the production of radiolytic O$_2$ and H$_2$O$_2$, and abiotic redox chemistry in the ocean and rocky core, contribute to chemical disequilibria that could support metabolic processes in Enceladus' ocean. We consider three possible cases for ocean redox chemistry: Case I in which reductants are not present in appreciable amounts and oxidants accumulate over time, and Cases II and III in which aqueous reductants or seafloor minerals, respectively, convert O$_2$ and H$_2$O$_2$ to SO$_4^{2-}$ and ferric oxyhydroxides. We calculate the upper limits on the concentrations of oxidants and chemical energy available for metabolic reactions in all three cases, neglecting additional abiotic reactions. For all three cases, we find that many aerobic and anaerobic metabolic reactions used by microbes on Earth could meet the minimum free energy threshold required for terrestrial life to convert ADP to ATP, as well as sustain positive cell density values within the Enceladus seafloor and/or ocean. These findings indicate that oxidant production and oxidation chemistry could contribute to supporting possible life and a metabolically diverse microbial community on Enceladus., Comment: Accepted to Icarus 2 December 2020

Titan's ionosphere contains a plethora of hydrocarbons and nitrile cations and anions as measured by the Ion Neutral Mass Spectrometer and Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft. Data from the CAPS Ion Beam Spectrometer (IBS) sensor have been examined for five close encounters of Titan during 2009. The high relative velocity of Cassini with respect to the cold ions in Titan's ionosphere allows CAPS IBS to function as a mass spectrometer. Positive ion masses between 170 and 310 u/q are examined with ion mass groups identified between 170 and 275 u/q containing between 14 and 21 heavy (carbon/nitrogen/oxygen) atoms. These groups are the heaviest positive ion groups reported so far from the available in situ ion data at Titan. The ion group peaks are found to be consistent with masses associated with Polycyclic Aromatic Compounds (PAC), including Polycyclic Aromatic Hydrocarbon (PAH) and nitrogen-bearing polycyclic aromatic molecular ions. The ion group peak identifications are compared with previously proposed neutral PAHs and are found to be at similar masses, supporting a PAH interpretation. The spacing between the ion group peaks is also investigated, finding a spacing of 12 or 13 u/q indicating the addition of C or CH. Lastly, the occurrence of several ion groups is seen to vary across the five flybys studied, possibly relating to the varying solar radiation conditions observed across the flybys. These findings further the understanding between the low mass ions and the high mass negative ions, as well as with aerosol formation in Titan's atmosphere.

Observations with Cassini's Electron Spectrometer discovered negative ions in Titan's ionosphere, at altitudes between 1400 and 950 km. Within the broad mass distribution extending up to several thousand amu, two distinct peaks were identified at 25.8-26.0 and 49.0-50.1 amu/q, corresponding to the carbon chain anions $CN^-$ and/or $C_2H^-$ for the first peak and $C_3N^-$ and/or $C_4H^-$ for the second peak. In this study we present the spatial distribution of these low mass negative ions from 28 Titan flybys with favourable observations between 26 October 2004 and 22 May 2012. We report a trend of lower densities on the night side and increased densities up to twice as high on the day side at small solar zenith angles. To further understand this trend, we compare the negative ion densities to the total electron density measured by Cassini's Langmuir Probe. We find the low mass negative ion density and the electron density to be proportional to each other on the dayside, but independent of each other on the night side. This indicates photochemical processes and is in agreement with the primary production route for the low mass negative ions being initiated by dissociative reactions with suprathermal electron populations produced by photoionisation. We also find the ratio of $CN^-/C_2H^-$ to $C_3N^-/C_4H^-$ highly constrained on the day-side, in agreement with this production channel, but notably displays large variations on the nightside., Comment: 11 pages, 5 figures

During March-April 2002, while between the orbits of Jupiter and Saturn, the Cassini spacecraft detected a significant enhancement in pickup proton flux. The most likely explanation for this enhancement was the addition of protons to the solar wind by the ionization of neutral hydrogen in the corona of comet 153P/Ikeya-Zhang. This comet passed relatively close to the Sun-Cassini line during that period, allowing pickup ions to be carried to Cassini by the solar wind. This pickup proton flux could have been further modulated by the passage of the interplanetary counterparts of coronal mass ejections past the comet and spacecraft. The radial distance of 6.5 Astronomical Units (au) traveled by the pickup protons, and the implied total tail length of >7.5 au make this cometary ion tail the longest yet measured., Comment: Submitted to Icarus

Some of the major discoveries of the recent Cassini-Huygens mission have put Titan and Enceladus firmly on the Solar System map. The mission has revolutionised our view of Solar System satellites, arguably matching their scientific importance with that of their planet. While Cassini-Huygens has made big surprises in revealing Titan's organically rich environment and Enceladus' cryovolcanism, the mission's success naturally leads us to further probe these findings. We advocate the acknowledgement of Titan and Enceladus science as highly relevant to ESA's long-term roadmap, as logical follow-on to Cassini-Huygens. In this white paper, we will outline important science questions regarding these satellites and identify the pertinent science themes we recommend ESA cover during the Voyage 2050 planning cycle. Addressing these science themes would make major advancements to the present knowledge we have about the Solar System, its formation, evolution and likelihood that other habitable environments exist outside the Earth's biosphere., Comment: White paper submitted in response to Voyage 2050 long-term plan in the ESA Science Programme

Remote sensing observations suffer significant limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. This impacts our knowledge of the formation of these planets and the physics of their atmospheres. A remarkable example of the superiority of in situ probe measurements was illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases' abundances and the precise measurement of the helium mixing ratio were only made available through in situ measurements by the Galileo probe. Here we describe the main scientific goals to be addressed by the future in situ exploration of Saturn, Uranus, and Neptune, placing the Galileo probe exploration of Jupiter in a broader context. An atmospheric entry probe targeting the 10-bar level would yield insight into two broad themes: i) the formation history of the giant planets and that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. An atmospheric probe could represent a significant ESA contribution to a future NASA New Frontiers or flagship mission to be launched toward Saturn, Uranus, and/or Neptune., Comment: 27 pages, 9 figures, White Paper submitted in response to ESA's Call for Voyage 2050 Science Themes. arXiv admin note: substantial text overlap with arXiv:1708.00235

In order to examine how the terrestrial life emerged, a number of laboratory experiments have been conducted since the 1950s. Methane has been one of the key molecules in these studies. In earlier studies, strongly reducing gas mixtures containing methane and ammonia were mainly used to simulate possible reactions in primitive Earth atmosphere, and amino acids and other organic compounds were detected. Since the primitive Earth atmosphere was estimated to be less reducing, contribution of extraterrestrial organics to the origin of life is considered quite important. Extraterrestrial organic chemistry has been experimentally and theoretically studied intensively, including laboratory experiments simulating interstellar molecular reactions. Endogenous and exogenous organics should have been supplied to the primitive ocean. Now submarine hydrothermal systems are considered one of the plausible sites of generation of life. Experiments simulating submarine hydrothermal systems where methane played an important role are now intensively being conducted. We have recognized the importance of such studies on possible reactions in other solar system bodies to understand the origins of life. Titan and other icy bodies, where methane plays significant roles, are especially good targets to be studied. In the case of Titan, not only methane-containing atmospheres but also liquidospheres composed of methane and other hydrocarbons have been used in simulation experiments. This paper summarizes experiments simulating various terrestrial and extraterrestrial environments, and possible roles of methane in chemical evolution are discussed.